I think the majority of the responses in this thread are irrelevant: if the customer didn’t sign they agreed to be charged for the fuel, this charge must be removed. Same as the dealer doesn’t charge for the electricity or labour to clean the car and pump up the tyres before handling the vehicle to the customer.
I have been thinking the same since first replying, it’s a bit of a non story really. Just agree to pay it (I normally ask for fuel to be put in before collection) or ask for them to supply with a minimum amount in the tank. Job done! I guess the story is more about why does the supplier not provide say £20 fuel without charge, which would be good PR but they are not obliged to do and often dont.